1. Field of the Invention
This invention relates generally to improvements in methods for forming three-dimensional objects from a fluid medium. More particularly, the invention relates to a new and improved stereolithography method involving the application of enhanced stereolithographic curing techniques to reproduce three-dimensional objects described by the input data design more accurately and economically from photocurable polymers.
2. Description of the Relevant Art
Stereolithography represents an expeditious way to quickly make complex or simple parts without conventional tooling. The basic stereolithographic process and apparatus to practice the process are described in U.S. Pat. No. 4,575,330 assigned to the assignee of the present invention. Since this technology depends on using a computer to generate its cross-sectional patterns, there is a natural data link to CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, curl and other distortions, as well as resolution, accuracy and difficulties in producing certain object shapes.
Objects built using stereolithography have a tendency to distort from their CAD designed dimensions. This distortion may or may not appear in a specific object, based on how much stress is developed by the specific cure parameters and on the ability of the object to withstand stress. The stress that causes distortion develops when material that is being converted from liquid to solid comes into contact with and bonds to previously cured material. When material is converted from liquid to solid, it shrinks slightly. All stereolithography resins undergo shrinkage when cured. The shrinkage will vary with the type of resin, the temperature change as a result of rate of exposure, and other related factors. This shrinking causes stress and has two primary physical causes: (1) density of the liquid is less than that of the solid plastic; and (2) the chemical reaction that causes the change of state is strongly exothermic, causing the curing material to thermally expand and contract.
Certain sections of an object will be able to resist stresses without any apparent warp, that is where stress is at a tolerable level. On the other hand, other sections may distort considerably as the stress and structural strength balance each other. Since stress is caused by contact between curing material and cured material, it can be propagated along the entire length of contact between the curing line and cured material. Most contact of curing to cured material occurs from one layer to the next as opposed to along a single layer. This implies most distortions will be vertical in nature as opposed to horizontal. Further, large features in the XY plane tend to shrink more than smaller features in the XY plane. Since shrinkage is a fixed volumetric percentage, when a part transitions from a large feature to a small feature along the Z direction, the difference in shrinkage values produces a pronounced discontinuity on the external surface that is a geometric imperfection more commonly known as the differential shrinkage effect.
Differential shrinkage is more pronounced in parts transitioning from a large feature to a small or thin feature. The greater the difference in the sizes of the features, the greater is the differential shrinkage. Typically the object being built has its largest portion cured in what is known as a hatch or fill pattern. Prior techniques of curing the build parts have experienced the greatest shrinkage or distortion in the largest portion of the part where the hatching or fill typically occurs and not in the border layers. The main part area where the shrinkage occurs will have flaws and surface anomalies. Prior curing methods utilizing multiple border cures have cured the borders to these central or main part areas by curing outer border portions first and then curing successive boundary passes inwardly toward the central areas. This method leaves the final joining of the border to the central hatched or filled area with the surface flaws or anomalies in the central hatched or filled area uncorrected. This results in inaccurate reproductions of the input data in the 3-dimensional object being formed. Alternatively, directly joining the border to the central hatched or filled part area results in the borders being distorted as the curing of the central hatched or filled area continues due to shrinkage of the larger central or main part area. Further exacerbating the distortion problem with the advent of laser technology advancements is the increased use of higher power lasers to cure the photocurable material, resulting in faster curing and accelerated heat generation and heat build-up in the cured parts over less time. Attempts to compensate for this increased heat build-up require the use of lowered power and smaller spot size, both of which are counter to the benefits of the higher powered lasers faster speed and greater curing ability. Other attempts to compensate for this problem have used increased delay times between component curing to reduce the effects of part shrinkage and the concomitant part distortion resulting form that shrinkage. Discussions of the ability to perform this type of curing are described in U.S. Pat. Nos. 6,103,176 and 5,902,538, both assigned to the assignee of the present invention. Discussion of techniques to reduce differential shrinkage, including the use of a delay after exposure of at least a portion of a cross-section or lamina are presented in U.S. Ser. No. 09/246,504 filed Feb. 8, 1999 and assigned to the assignee of the present invention.
Therefore, there is a need for a technique to reduce differential shrinkage and distortion in parts fabricated using stereolithography without affecting the overall build or fabrication time by excessive or increased delay times to permit curing of components to occur.
It is an aspect of the present invention that a method of producing stereolithographic parts is disclosed that uses a curing technique which reduces or controls differential shrinkage to an acceptable level in the building of parts.
It is another aspect of the present invention that shrinkage during the building of stereolithographic parts is controlled at the interface between the main hatched or filled part area and the part border.
It is a further aspect of the invention that delay time between component vectors and true CAD boundary scans to reduce shrinkage from resin or build material curing is minimized or eliminated.
It is a feature of the present invention that a build style is used that permits a sufficient delay period after curing the main hatched or filled area to permit shrinkage to occur and combines the delay period with attaching the part border directly to the hatched or filled area.
It is another feature of the present invention that the attaching of the part border directly to the hatched or filled area in successive boundary curing passes is accomplished from the nearest border area directly adjacent to the main hatched or filled area to the outside or furthest portion from the hatched or filled area.
It is yet another feature of the present invention that the technique can employ the use of both large and small laser beam spot sizes to cure both the hatched or filled main part areas and the border areas.
It is yet another feature of the present invention that the curing technique provides liquid photocurable material to fill all of the shrunken areas of the main hatched or filled area during the multiple boundary drawings.
It is still a further feature of the invention that supports for each component of a part are drawn between subsequent component vector scanning and true CAD boundary drawing.
It is still another feature of the present invention that the vector types which are drawn by the laser are ordered to reduce the delay time between component draws and concurrently accomplish as much build as possible and reduce or eliminate decreases in the overall stereolithography system throughput.
It is an advantage of the present invention that the stereolithographic method of curing the photocurable material reduces distortion in the final part and preserves the flatness and integrity of the part borders.
It is another advantage of the present invention that the curing of the border areas in multiple boundary drawings from the portion closest adjacent to the main part area to the portion furthest from the main part area reduces the surface anomalies and shrinkage in the final stereolithographic part.
It is yet another advantage of the present invention that the speed of the build of a stereolithographic part is not sacrificed to an unacceptable level to achieve reduced differential shrinkage in the part.
It is still another advantage of the present invention that the physical property of tensile elongation at break of the part is not reduced.
These and other aspects, features and advantages are achieved in the method of the present invention by pausing or delaying for a period of time after curing the main hatched or filled area of the part to permit shrinkage to occur and then curing the borders of the part adjacent the main hatched or filled area from the inside out in multiple boundary drawing passes to produce a part that is accurately reflective of the input data and with less distortion, but increased tensile elongation at break.